MARC is the great-granddaddy of data exchange formats, based on ANSI standard Z39.2, American National Standard for Bibliographic Information Exchange and ISO 2709: 1996, Information and Documentation - Format for Information Exchange. MARC was first rolled out publically by OCLC in 1967. Since then, it has become the backbone for metadata exchange among libraries and archives around the world. At first there were various, flavours of MARC - however after a decade of negotiation and standards alignment, in 1977 UNIMARC was developed to allow conversion between various regional formats. Then, after many years of descreptive cooperation, the American USMARC & Canadian CAN/MARC standards were integrated into MARC 21 (detailed below) in 1999. This finally allowed easy standardized sharing and exchange of cataloguing data for libraries and institutions throughout North America. Negotiations are underway to bring UKMARC under this umbrella in the near future.

MARC actually contains five data formats: bibliographic, authority, holdings, community information and data classification - the last four are, broadly speaking, technical elements used internally by libraries, archives and other institutions to track classification, usage and ensure record integrity. The Bibliographic format however, makes up what the public sees. As you can see below, the records would be expansive if all the fields & text were used. Instead, as Sylvar outlines above, only the three digit tags applicable to a record are used. Each field then has specifically defined subfields for the description of each data element.

For example, the Publication field is tag 260, and has three subfields: a) place of publication, b) name of publisher and c) date of publication. Each subfield is within a tag is marked off by a delimiter (usually $, # or _ characters) so the catalogue software knows where the data for each field starts and ends. Hence, the 260 ## $aBoston :$bLittle Brown,$c1989. A bit confusing at first, once you know the major fields and subfields, it’s not so hard to see how it all fits.

So. Need to get your data organized, regardless of format? Here's how the descriptive/bibliographic section breaks down:
00X Control fields - General information

001 Control number
003 Control number identifier
005 Date and time of latest transaction
006 Linking field
007 Physical description fixed field
008 Fixed fields

210 Abbreviated key title
222 Key title
240 uniform title
241 Romanized title
242 Translation of title by cataloguing agency
243 Collective uniform title
245 Title statement
246 Varying form of title
247 Former title or title variation